Circuit and method for deskewing variable supply signal paths

Abstract

A circuit and method for deskewing signals by using cross power supply logic paths to compensate for delays created by power supplies operating at different voltages. A first replica circuit operating at a first supply voltage is placed in series with a first signal operating at a second supply voltage. A second replica circuit operating at the second supply voltage is placed in series with a second signal having a skew difference from the first signal and operating at the first supply voltage. The replica circuits generally have a scale factor which is generally a fraction of the equivalent driver circuits associated with the particular output signals. As a result, the present invention will deskew arbitrary power supply differences. By matching delays, the replica and true circuits provide the same delay. As a result, the sum of the delays for all the blocks in each path will be constant which maintains a desired skew difference between the output signals.

I claim: 
     
       1. An apparatus comprising: a first circuit operating at a first supply voltage configured to present a first output in response to an input;   a second circuit operating at a second supply voltage configured to present a second output in response to said input;   a third circuit coupled between said input and said first circuit, wherein said third circuit operates at said second supply voltage;   a fourth circuit coupled between said input and said second circuit, wherein said fourth circuit operates at said first supply voltage, wherein said third and fourth circuits provide compensation such that said first and second outputs have a minimum skew;   a fifth circuit coupled between said first and third circuits, wherein said fifth circuit is configured to provide (i) level shifting between said first and third circuits and (ii) an adjustable capacitive load; and   a sixth circuit coupled between said first and third circuits, wherein said sixth circuit is configured to provide (i) level shifting between said second and fourth circuits and (ii) an adjustable capacitive load.   
     
     
       2. The apparatus according to claim 1 wherein said third and fourth circuits have a smaller area by a ratio of 1/N with respect to said first and second circuits. 
     
     
       3. The apparatus according to claim 1 wherein: said first circuit and said fourth circuit operate at one supply voltage; and   said second circuit and said third circuit operate at another supply voltage, wherein said supply voltages are different.   
     
     
       4. The apparatus according to claim 1 wherein said first and second circuits each comprise a driver and pad circuit. 
     
     
       5. The apparatus according to claim 2 wherein said third and fourth circuits each comprise a driver and pad circuit reduced in size by said ratio with respect to said first and second circuits. 
     
     
       6. The apparatus according to claim 1 wherein said: fifth circuit comprises one or more inverters; and   said sixth circuit comprises one or more inverters.   
     
     
       7. The apparatus according to claim 1 comprising: a plurality of said first and second circuits;   a plurality of said third and fourth circuits; and   a plurality of said first and second outputs, wherein one of said first and second circuits, one of said third and fourth circuits and one of said first and second outputs operate at a third supply voltage, wherein each of said first and second outputs has a minimum skew.   
     
     
       8. An apparatus comprising: first means for generating a first output operating at a first supply voltage in response to an input;   second means for generating a second output operating at a second supply voltage in response to said input;   first means for delaying said first output, said first delay means coupled between said input and said first generator means, wherein said first delay means operates at said second supply voltage;   second means for delaying said second output, said second delay means coupled between said input and said second generator means, wherein said second delay means operates at said first supply voltage, wherein said first and second delay means provide compensation such that said first and second outputs have a minimum skew;   means for level shifting between said first generator means and said first delay means, comprising a first adjustable capacitive load; and   means for level shifting between said second generator means and said second delay means, comprising a second adjustable capacitive load.   
     
     
       9. The apparatus according to claim 8 wherein said first and second delay means have a smaller area by a ratio of 1/N with respect to said first and second generator means. 
     
     
       10. The apparatus according to claim 8 wherein: said first generator means and said second delay means operate at one supply voltage; and   said second generator means and said first delay means operate at another supply voltage.   
     
     
       11. The apparatus according to claim 8 wherein said first and second level shifting means each further comprise an adjustable capacitive load. 
     
     
       12. A method for deskewing outputs of first and second circuits respectively operating at first and second supply voltages comprising the steps of: delaying one of said outputs a sufficient time to deskew said outputs, wherein said delay corresponds to a propagation delay of one of said supply voltages;   adjusting a voltage level of one of said outputs using a variable capacitive load to compensate for said propagation delay;   delaying another one of said outputs by a second delay, wherein said second delay corresponds to a second propagation delay of another one of said supply voltages; and   adjusting a voltage level of another one of said outputs using a variable capacitive load to compensate for said second propagation delay.   
     
     
       13. The method according to claim 12 wherein said level shifting steps each comprise the step of generating a fixed capacitive load. 
     
     
       14. The method according to claim 12 wherein said level shifting steps each comprise the step of generating a variable capacitive load. 
     
     
       15. The method according to claim 12 wherein said level shifting steps each comprise the step of: generating a fixed capacitive load.